One of the general requirements for a high performance analog circuit is a good bipolar current gain control. In order to obtain and control the required current gains, interface oxide materials (IFO) are widely used. In an NPN transistor, the base current (holes) should be controlled with a thin oxide, which has to break up during thermal processing. However in a PNP transistor, the base current, in this case electrons, has to be controlled by an interface oxide that does not break up and maintains its structure so as to ensure its barrier properties.
For vertical PNP transistors it is a basic requirement to have an interface oxide with a very good uniformity and a high thermal stability, and for this purpose IFOs currently in use have a thickness in a range of 3-8 Å. However, IFOs as thin as 3-8 Å are not stable over more than one hour after they have been deposited during fabrication of the transistor. With specific surface treatment processing of the IFO, there may be a small increase in surface stability of the IFO, due to the formation of hydrogen bonds after cleaning, but over 2-3 hours the oxide starts to grow almost linearly with time depending on atmospheric and lighting conditions around the wafers upon which the PNP devices are being fabricated.
IFOs in the thickness range of 3-8 Å are also susceptible to break down upon post thermal treatment. The IFO has to have good stochiometry to withstand thermal cycles without the breakup effect seen in NPN processes, so as to avoid degradation effects and ensure reliable PNP characteristics. To achieve this stochiometry and quality, the process window has to be shifted to higher IFO temperatures, with reduced pressure and reduced oxygen partial pressures so as to reduce the oxidation rate to meet the required IFO thicknesses. This means that the process window becomes very narrow.